1. Field of the Invention
The present invention relates to an image forming apparatus including a fixing section, and a method of forming an image.
2. Description of Related Art
In general, an electrophotographic image forming apparatus (such as a printer, a copy machine, and a fax machine) is configured to irradiate (expose) a charged photoconductor with (to) laser light based on image data to form an electrostatic latent image on the surface of the photoconductor. The electrostatic latent image is then visualized by supplying toner from a developing device to the photoconductor (image carrier) on which the electrostatic latent image is formed, whereby a toner image is formed. Further, the toner image is directly or indirectly transferred to a sheet through an intermediate transfer belt, followed by heating and pressurization for fixing, whereby an image is formed on the sheet.
The fixing section includes a fixing side member (for example, fixing belt) disposed on the fixing surface (the surface on which a toner image has been formed) side of sheets, and a back side supporting member (for example, pressure roller) disposed on the back side (the surface opposite the fixing surface) of sheets. The back side supporting member is brought into pressure contact with the fixing side member, thus forming a fixing nip for conveying sheets in a tightly sandwiching manner. In the following description, the fixing side member and the back side supporting member are collectively referred to as a fixing member.
In the above-mentioned image forming apparatus, in the case where images are formed continuously on a large number sheets of the same size, when the relative position between the sheet and the fixing nip in a sheet width direction (a horizontal scanning direction or a direction orthogonal to a sheet conveyance direction) is always the same, edges of lateral end portions of the sheet form edge scars on the fixing member (fixing side member, in particular). Such edge scars lead to decrease in image quality.
As a technique for preventing the edge scar, Japanese Patent Application Laid-Open No. 2006-91224, for example, discloses an image forming apparatus in which a fixing section is translated in a sheet width direction in order to periodically change the relative position between the sheet and a fixing nip in the sheet width direction.
In addition, since variously-sized sheets are typically used in image formation, a sheet passing portion (a portion through which a sheet is passed) and a non-sheet passing portion (a portion through which a sheet is not passed, or a portion near lateral end portions) in a fixing member are differ from sheet to sheet. For example, a non-sheet passing portion for a small-sized sheet may be a sheet passing portion for a large-sized sheet. Meanwhile, the non-sheet passing portion of the fixing member has a temperature higher than that of the sheet passing portion thereof since heat of the non-sheet passing portion does not transferred to sheets. Therefore, when an image formation is performed on a large-sized sheet after an image formation is performed on a small-sized sheet, the temperature distribution of the fixing member in the sheet width direction becomes non-uniform, thus causing uneven fixation, wrinkles or hot offset due to excessive heating.
As a technique for preventing temperature rise at an end portion as the non-sheet passing portion of a fixing member during a fixing operation, Japanese Patent Application Laid-Open No. 2009-175344, for example, discloses an image forming apparatus in which the fixing section is moved to an end side or the other end side in the width direction in accordance with the temperature of the fixing member.
In the following description, the motion of the fixing section in one direction is referred to as “movement” of the fixing section, and in particular, the periodic reciprocating motion of the fixing section is referred to as “translation” of the fixing section.
In addition, Japanese Patent Application Laid-Open No. 2011-118287 discloses that a combination of translation of a fixing section as measures against edge scars and moving of the fixing section in accordance with the temperature of the fixing member is applicable.
However, conventionally, an image forming apparatus which can prevent formation of edge scars on a fixing member and can uniformize the temperature of the fixing member has not been proposed.
Specifically, in Japanese Patent Application Laid-Open No. 2006-91224, the image forming apparatus only translates the fixing section, and the temperature distribution of the fixing member is not taken into account. Therefore, the temperature of the fixing member (in particular, the temperature of an end portion) may become non-uniform when the fixing section is translated.
Since the image forming apparatus in Japanese Patent Application Laid-Open No. 2009-175344 moves the fixing section to an end side or the other end side in accordance with the temperature of the fixing member, it is recognized that the sheet passing region is thereby changed, making it possible to prevent formation of edge scars. However, since image formations are sequentially performed in such states, formation of edge scars cannot be sufficiently prevented.
In addition, the techniques according to Japanese Patent Application Laid-Open Nos. 2006-91224 and 2009-175344 are both intended to be performed during the non-sheet passing period during which sheets do not pass through the fixing nip. Therefore, if the both techniques are used at the same time, only one of the techniques is prioritized, and the effect of the other technique cannot be sufficiently obtained.
The image forming apparatus according to Japanese Patent Application Laid-Open No. 2011-118287 prioritizes moving of the fixing section based on the temperature of the fixing member on the premise that a certain temperature gradient is formed in the fixing member, and therefore the range of translation of the fixing section for preventing formation of edge scars is limited. In addition, in the image forming apparatus according to Japanese Patent Application Laid-Open No. 2011-118287, variation of the temperature distribution of the fixing member (in particular, the temperature of a non-sheet passing portion) with time is not taken into account, and therefore it is difficult to uniformize the temperature of the fixing member.